Generally, a scroll structure used in a turbine is a structural member which is disposed in front of a first stator blade (inflow side of working fluid) or is disposed behind a last rotor blade (outflow side of working fluid) in the turbine using high temperature steam or high temperature gas as the working fluid, and the working fluid flows through the scroll structure. The scroll structure is formed by sheet-metal welding or casting.
Among the scroll structures, in the scroll structure through which heated working fluid flows in or out, in order to satisfy material strength required for the scroll structure, an outer peripheral surface of the scroll structure is sprayed with cooling fluid to lower the temperature of the scroll structure.
It is necessary to construct a thermal insulation structure for preventing the temperature of the scroll structure from rising or for preventing heat of working fluid flowing through the scroll structure from being radiated or absorbed, but it is also known to attach a thermal insulation material inside the structural member to block heat (see Japanese Unexamined Patent Application, Publication No. H9-14576 for example).
However, according to the thermal insulation structure described in Japanese Unexamined Patent Application, Publication No. H9-14576, it is not assumed that the thermal insulation structure is formed into a three-dimensional shape like the scroll structure, and there is a problem that it is difficult to construct such a thermal insulation structure.
In the above thermal insulation structure, an outer casing constituting the structural member and an inner wall which pushes a heat insulator against the outer casing are fixed to each other. Therefore, when a thermal extension amount of the outer casing and a thermal extension amount of the inner wall are different from each other, a difference between the thermal extension amounts is not absorbed, and there is an adverse possibility that the thermal insulation structure is damaged.
Since a thermal insulation material is disposed between shelves which are fixed to the outer casing by welding, there is a problem that the shelves are required to be disposed in order to construct the thermal insulation structure.
The thickness of the above thermal insulation structure is determined by the height of the shelves, and there is thus a problem that it is difficult to adjust the thickness of the thermal insulation structure.
When cooling fluid is sprayed to lower the temperature of the scroll structure, a system for supplying the cooling fluid is separately required. Extracted working fluid is used as the cooling fluid in many cases, and there is a problem that efficiency of a turbine having the scroll structure or a plant having the turbine is deteriorated although the efficiency differs depending upon the temperature condition or structure condition.
When the thermal insulation structure is provided on an inner surface of the scroll structure, a portion of the thermal insulation structure is peeled off and drops, and flows into a row of blades of the turbine portion disposed downstream due to flow of the working fluid, which damages a rotor blade or a stator blade, and the thickness of a tube is reduced due to erosion in some cases.